FIG. 18 is a cross-sectional view of a waveguide portion of a distributed feedback semiconductor laser widely used for optical communication that is disclosed in Japanese Unexamined Patent Publication No. Hei-111383. FIG. 18 is a cross-section taken parallel to the path of the laser light wherein a diffraction grating 6, a lower cladding layer 2 of p type InGaAsP, an active layer 3 of InGaAsP, an upper cladding layer 4 of n type InP, and a contact layer 5 of n.sup.+ type InGaAsP are successively disposed on a p type InP substrate 1. Further, a .lambda./4 phase shift portion 7 is included in the diffraction grating.
When a current flows in the semiconductor laser, light is generated in the active layer 3 due to the recombination of charge carriers. Since the active layer is interposed between the lower cladding layer 2 and the upper cladding layer 4, which have a smaller refractive index than the active layer, the light generated propagates in the active layer 3. The propagated light is reflected at the facets, i.e., end surfaces, and light components having a common phase mutually increase the intensity of light. Further, since the diffraction grating 6 has a distributed feedback structure, light propagating in a substantially uniform mode can be obtained. In addition, since the diffraction grating 6 has the .lambda./4 phase shift portion 7, a phase inversion occurs at the .lambda./4 shift portion 7 during the propagation of light, and light having a single wavelength is emitted.
In the conventional semiconductor laser as shown in FIG. 18, the intensity of the light is extremely strong at the .lambda./4 phase shift portion 7 as shown by the character A in FIG. 20, and so-called hole burning easily occurs, whereby the mode of light may become unstable at a high power light output. In order to suppress hole burning, a structure in which the amplitude of the diffraction grating 6 gradually decreases in the direction of the .lambda./4 phase shift portion 7 has been used to reduce the reflectance of light, as shown in FIG. 19. In this structure, an increase of the light intensity at the .lambda./4 phase shift portion 7 can be prevented as shown by the line B in FIG. 20.
In order to produce the structure shown in FIG. 19, the p type InP substrate 1 is etched so that the rate of etching is partially or continuously changed whereby the diffraction grating 6 has an amplitude that partially or continuously changes. However, it is very difficult to partially or continuously change the etching rate of the p type InP substrate 1 to form the diffraction grating 6 with partially or continuously changing amplitude. Generally, the pitch of the diffraction grating is in a range of about 0.2-0.3 .mu.m. This structure makes the formation of the diffraction grating 6 still more difficult.
Besides the diffraction grating 6 shown in FIG. 19, there has been proposed an embedded diffraction grating as shown in FIGS. 21 and 22 in Japanese Unexamined Patent Publication No. Hei 2-307287. FIG. 21 shows a semiconductor laser in which, on a p type InP substrate, a buffer layer 8 of p type InP, an active layer 3 of InGaAsP that has a smaller band gap energy than the buffer layer 8, a barrier layer 9 of n type InP that has a larger band gap energy than the active layer 3, an embedded diffraction grating 10 of n type InGaAsP that has a larger band gap energy than the active layer 3 but a smaller band gap energy than the barrier layer 9, an upper cladding layer 4 of n type InP which has the same composition as the barrier layer 9, and a contact layer 5 of n.sup.+ type InGaAsP are successively disposed. The diffraction grating 10 has a constant pitch .LAMBDA., a constant width W, and a constant amplitude d.sub.0. A .lambda./4 phase shift portion 7 is included in the diffraction grating.
FIG. 22 shows a semiconductor laser in which a p type InP substrate 1, an embedded diffraction grating 10 of p type InGaAsP that has a smaller band gap energy than the p type InP substrate 1, a barrier layer 90 of p type InP that has the same composition as the InP substrate 1, an active layer 3 having a smaller band gap energy than the embedded diffraction grating 10, an upper cladding layer 4 of n type InP, and a contact layer 5 of n.sup.+ type InGaAsP are successively disposed. The embedded diffraction grating 10 has a constant pitch .LAMBDA., a constant width W, and a constant amplitude d.sub.0. A .lambda./4 phase shift portion 7 is included in the diffraction grating.